by Andrew Greenspon
figures by Hannah Zucker

Picture yourself as the Curiosity Rover, which landed on Mars in 2012. You’ve just arrived on Mars after an eight-month journey from Earth. You begin traveling across the Gale crater toward Mount Sharp, 18,000 feet higher than the floor of the crater. On the way, you take images with a high-resolution camera. There, you find a slab of rocks made of pieces of gravel smooshed together. The size and shape of these stones indicates that they were likely formed by a river stream on the surface of Mars some time in the ancient past. Could such a stream have contained extraterrestrial life long ago?

Water has been a major driver of exploration of the solar system. Essential to the formation of all known life, evidence of stable liquid water elsewhere could indicate an environment beyond Earth that could sustain the formation of extraterrestrial life. Additionally, if humans are to ever have a long-term presence on an extraterrestrial body, there will need to be native sources of water at those locations. Water away from Earth would be necessary for drinking, breathing, and producing fuel to operate spacecraft or spacebase systems. But how exactly do we find water in such far off places without going there ourselves?

How do we find water in the solar system?

While on Earth we can see water and handle it with our bare hands, detecting water or finding evidence of past water from far away is much more difficult. Optical telescopes that collect visible light and provide visual images of distant bodies only give us some indication of the brightness and large-scale shapes and structures of large regions. Brighter regions, especially near the north or south pole of a planet or moon could indicate reflections of frozen water (think of how shiny ice can be when you walk down the street in winter). However, when it’s cold enough, even carbon dioxide (a gas at room temperature on Earth) forms a reflective solid. As such, optical telescopes alone cannot confirm the presence of water.

Optical telescopes can also give some indication of mountains and valleys on other planets, but the large distance from Earth makes it very difficult to determine the size and structure of smaller geological features, like those that may have been formed with the help of flowing water. Placing a camera closer to a planet via an orbiting spacecraft allows scientists to collect much higher resolution images of the surface. Spacecraft that can land and even drive on the planet’s surface (called rovers), allow humans to “move” around a planet to look at the size and shape of rocks ranging from larger boulders to tiny pebbles.

Figure 1: Extraterrestrial exploration. There are many different ways that we search for water on extraterrestrial bodies, such as planets, moons, and asteroids. These include landers, rovers, and various varieties of telescopes.

In addition to camera images, scientists also indirectly understand what material is on a planet by measuring the reflectance of light off the surface. Different materials, including water, absorb and reflect different wavelengths of light – both visible light that humans see as different colors, and light that we cannot see with our eyes. The latter includes ultraviolet (shorter wavelengths than visible that can give you a sunburn) and infrared (longer wavelengths than visible that can be used to heat food in your microwave). The relative intensity of reflection of different wavelengths is together called a spectrum, which is measured to narrow down the possible range of materials on the surface of an extraterrestrial body. 

Additional instruments can be designed to detect other wavelengths of light or elementary particles like neutrons emanating from the surface of an extraterrestrial body. These signals can be further interpreted to detect atomic elements such as hydrogen (H), which is one component of water. Often, the data from multiple instruments and camera images must be put together to actually determine what’s on or even just below the surface.

Additionally, landers and rovers can collect samples from the surface of the planet to be placed in an analysis chamber that can determine the chemical composition and types of minerals in the sample, such as clay minerals that likely formed in a liquid water environment. The capabilities of such chambers are limited by size, power requirements, and remote control from Earth. (On Earth, multiple samples could be moved from machine to machine and modified with human hands that have more dexterity than a robot arm.) Spacecraft can also collect a sample and return it to Earth for more detailed analysis. However, such sample return missions are very expensive and difficult to perform owing to the need to land on the surface of an extraterrestrial body, escape its gravitational pull, and return to Earth.

Given the difficulty of confirming the presence of water on extraterrestrial bodies, even with all these tools, how do we decide where to look first?

The early search for water on Mars

Mars was one of the first extraterrestrial bodies that captured human attention due to a belief in liquid water flowing across the surface with accompanying life. In the early 1780s, the astronomer William Herschel calculated the rotation period of Mars and the planet’s tilt to be similar to Earth’s, leading him to conclude “the analogy between Mars and the Earth is, perhaps, by far the greatest in the whole solar system” and “that its inhabitants probably enjoy a situation in many respects similar to ours.” Images of Mars from telescopes showed changing brightness on the planet over time, which some scientists believed could be water or vegetation growing and receding seasonally, as happens on Earth. 

Despite these early hopes in the capacity of Mars to house both water and life, scientific evidence indicated otherwise. By the early 20th century, most astronomers agreed that Mars must be much drier and colder than Earth, and that no complex forms of life could survive there. Light collected from the planet showed primarily a carbon dioxide atmosphere with no detectable amounts of oxygen or water vapor at the time. (Note that modern data shows trace amounts of oxygen and water vapor.) Because of the cold temperature and thin atmosphere on Mars (0.6% that of Earth), liquid water is unstable almost everywhere on the planet’s surface. Water-ice (water in solid form) directly converts into water vapor when warmed up enough.

Modern Martian observations up-close

Despite evidence from afar of the cold and dry nature of Mars, scientists still sent spacecraft to Mars for a closer look at the planet and to search for evidence of any signs of present or past life on the planet. The first spacecraft to orbit and land on Mars in the 1970s, Mariner 9, Viking 1, and Viking 2, collected images of geological features that looked like dry river beds and canyons similar to those seen on Earth. Scientists believe such features all over the Martian surface indicate a history of water on Mars billions of years in the past. 

Figure 2: A history of Martian exploration. This image shows a brief timeline of Mars missions that provided key evidence in the search for water in the past and present on the red planet. See here for a more detailed timeline of all missions to Mars.

Many more satellites, landers, and rovers have been sent to Mars since then, with a variety of instruments that have detected a large amount of water-ice just below the surface across the planet. The Curiosity Rover has also collected from the Martian surface clay minerals that form in the presence of fresh water, giving further evidence of past liquid water on Mars. 

In 2015, data collected from the Mars Reconnaissance Orbiter suggested that recurring dark streaks on the Martian surface were due to liquid brine (water mixed with salt) existing just below the surface during the warmer season on Mars (-10 F) that would make its way to the surface for a brief time. However, additional data in 2017 cast this hypothesis into doubt, suggesting instead that these dark streaks are primarily due to grains of sand and dust cascading down very steep slopes on the rugged planet. 

As mentioned above, liquid water is generally unstable on the Martian surface. However, it can exist under certain conditions below the surface. Specifically, in 2018 scientists using radar data from the European Space Agency’s Mars Express orbiter identified a brine liquid-water lake a mile below the southern ice cap of Mars. The high levels of salt and frigid temperatures would make it difficult for most known lifeforms to survive. However, even on Earth we have discovered organisms that thrive in salty, though not so cold, conditions. While these recent data give strong evidence of a form of liquid water on Mars, to find direct evidence of present or past life in that water, missions would have to be sent to drill into the Martian surface. Such drilling would be incredibly difficult, and there are no current public or private plans for such a mission. Additionally, scientists are hesitant to perform such missions out of fear of contaminating or destroying any possible life that might be detected. Last, if humans are to ever go to Mars, it will likely be easier for them to melt surface or near-surface ice for survival than to dig deep into the ground to purify this salt-water mixture.

Water closer to home?

In addition to Mars, the quest for water beyond Earth has also been focused on our closest extraterrestrial body, the Moon. Since the Moon lacks any significant atmosphere, liquid water cannot exist on its surface. Any water-ice exposed to sunlight quickly converts to water vapor and breaks down into oxygen and hydrogen. As such, water-ice would only be stable in areas of the moon that lie in a permanent shadow, such as near the poles and in craters. Only in August of 2018 did NASA confirm via the Moon Mineralogy Mapper (M3) instrument that there is water-ice on the surface of the moon near the poles.

Vast oceans of water in far off places

Beyond Mars and the Moon, scientists have hypothesized sources of water further afield from Earth. The large outer planets of our solar system, Jupiter and Saturn, have been viewable by telescope for centuries, as have many of their very large moons. As such, scientists have been fascinated by these extraterrestrial bodies for some time. Evidence suggests that Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, have huge subsurface oceans with a layer of ice covering their surfaces. The subsurface of these moons stays liquid through the gravitational forces of the nearby large planet and surrounding moons alternately squeezing and stretching the subsurface oceans, which generates heat through friction.

Spacecraft have explored both of these moons already. The Cassini spacecraft exploring Saturn and its moons has detected geysers of water being ejected from Enceladus, in addition to various chemicals that would be necessary for life. The Hubble Space Telescope from afar and the Galileo spacecraft up close to Jupiter have detected similar ejections of water vapor coming from the surface of Europa. Evidence suggests that other moons of Jupiter and Saturn such as Ganymede and Titan may have subsurface oceans as well. Asteroids in the asteroid belt also contain large amounts of water-ice that could be harvested if humans ever regularly travel beyond the inner Solar System

Let water be our guide

Evidence of various forms of water has been discovered in many places in our solar system and is hypothesized to exist in many more. These sources of water will be essential if humans ever can set up long-term, manned deep space missions. Additionally, the search continues for liquid water capable of sustaining life. That life may have existed long ago as in the case of Martian history, or it may exist in places we cannot reach yet such as the hypothesized subsurface ocean of Europa. In fact, a new satellite is planned to orbit Europa within the next decade to search for and study this possibly enormous body of water to find out if it may sustain life. As such, water, essential to human and all other known life, will continue to guide human exploration of the solar system and beyond. 

Andy Greenspon is a PhD candidate in Applied Physics at the School of Engineering and Applied Sciences.

Hannah Zucker is a PhD student in the Program in Neuroscience at Harvard University.

Cover image: Courtesy NASA/JPL-Caltech

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